Just one thing, frequent and regular maintenance. No magic potion can replace it.

Doesn't matter what concoction you use, (unless you are going to pack it away in cosmoline and wrap it in wax paper before stuffing it in an air-tight tube), if oxidation has begun it will always recur as long as air gets to it. The finish in that area has been compromised and will forever require frequent attention.

Handle all firearms with a pair of brown jersey gloves impregnated with oil, this will help eliminate finger prints and body oil from forming rust spots. Keep an oil-soaked rag handy and wipe your guns down every time you pick one up.